Elements, Mixtures, and Compounds Help (page 2)
Over time, scientists discovered that some matter is composed of pure chemicals. Pure substances are homogeneous and have certain unchanging chemical compositions. For example, a pure sample of highly condensed carbon, diamond, will always have the same crystalline structure. The repeating structural unit of diamond consists of eight atoms in cubic shapes. Using this cubic form and its highly symmetrical arrangement of atoms, diamond crystals form several different shapes. We will discuss this in more detail in Chapter 15. This cubic form and its light reflectivity make diamond one of the most desired substances on Earth.
Chemicals that have the same type of matter all through the sample are said to be pure elements . Oxygen, potassium, mercury, and nickel are pure elements.
During his research in 1789, Antoine Lavoisier defined an element as a substance that could not be decomposed by a chemical reaction into simpler substances. Lavoisier identified 33 elements that he thought were pure and indivisible. Of those 33, 20 of the 109 elements currently identified, are still considered pure elements.
An element is made up of a pure sample with all of the same kinds of atoms and cannot be further separated into simpler elements.
Mixtures can be separated into two or more substances manually. No chemical reaction is needed. In nature, salt water can be separated into its components of water and salt by allowing the water to evaporate. Mixtures are found in two forms: heterogeneous and homogeneous .
A heterogeneous mixture is one with physically separate parts that have different properties. An easy example is salt and pepper. A heterogeneous mixture has separate phases . A phase represents the number of different homogeneous materials in a sample. Salt is all one phase and pepper is one phase. They do not have a wide variety of characteristics, but are physically separate.
A homogeneous solution has one phase (liquid) but may have more than one component within the sample. Again, salt water is an example of a homogeneous mixture. It is the same throughout, but has two parts: water and salt. Figure 3.3 compares matter and its different parts.
Pure chemicals that can be broken down into simpler chemicals are known as compounds . Commonly, chemical compounds are made up of two elements in set proportions to each other. Water provides an easy example of a compound. It is composed of the elements hydrogen and oxygen. There are always two parts of hydrogen to one part oxygen in every molecule of water. If the water sample is from the sea or polluted, there may be other chemicals added, but basic water always has the same proportion of hydrogen to oxygen by mass.
In order to better understand how elements combine to become different kinds of matter, it is important to understand the idea of percent . The word, cent, comes from the Latin word centum which means one hundred. Percent stands for the number of parts of one material included in the total amount of another sample.
Scientists in all areas of study use the concept of percent to do their analyses. The general formula for percent looks like the following:
x / x total = n /100
Being able to calculate percent easily will make your laboratory experiments a lot less likely to give you problems. By starting out with the correct percentages and ratios of reactants, your chances of success (or at least a good grade) increase dramatically. Try some of the following examples.
Consider a rancher who has 100 horses: 24 are Appaloosas, 16 are Bays, 20 are Paints, and 40 are Palominos. What percent of Appaloosas does he have? He has 24 out of a hundred or 24%.
24/100 = n /100
n = 24%
On a different scale, consider a typical classroom of 30 people. In this group, 3 are interested in English and journalism, 12 are interested in medicine and science, 6 are interested in teaching and education, and the final 9 want to pursue art, communications, and film. Since 30 is the total number of people in the class, we treat them as the whole or 100%. To figure out the percent of people who want to pursue various areas of further study, you divide by 30.
3/30 = 0.1 0.1 × 100 = 10%
(Multiplying by 100 allows you to figure out what percent of the whole is given.) 10% of the people in the class want to study English and journalism.
12/30 = 0.4 0.4 × 100 = 40% (science and math)
6/30 = 0.2 0.2 × 100 = 20% (teaching and education)
9/30 = 0.3 0.3 × 100 = 30% (art, communications, and film)
In chemistry, percent is used to record the amounts of an element within the entire material or form.
Let’s look at the human body. It is composed of many different compounds in various amounts. In science fiction terms, humans are referred to as “carbon-based units” since the average human is composed of 18% carbon by mass out of all the elements in the body. Figure 3.4 gives an idea of the variety of elements that combine to keep our bodies healthy.
On a larger scale, the Earth is composed of many of the same elements found in our bodies as well as additional minerals and metals. Figure 3.5 lists the more plentiful elements in the Earth’s surface. The diversity of these elements supports and provides for millions of species of plants, animals, and minerals found on this planet. Without this large resource of diverse elements, or if it should swing far in the direction of one element over the others, our planet could become as barren as many others in our solar system.
One of the interesting things about our planet is the wide distribution of elements around the globe. Some of this diversity is due to the original formation of the continents and some is due to environmental factors, but the elements’ diverse properties play a huge role in making our world what it is today.
Without many different elements, life on Earth might still be just primordial ooze floating in pockets of sludge upon a primitive sea.
Practice problems for these concepts can be found at - Properties of Matter Practice Test